1. Field of the Invention
The present invention relates to an improved transmission rope structure, and more particularly to a transmission rope structure applicable to an image scanning device for driving the movable mirror modules thereof. The transmission rope is able to minimize the length deformation of the transmission rope during driving the movable mirror modules.
2. Description of the Prior Art
In a conventional image scanning device, in order to achieve the scanning function for a document to be scanned, there is provided with movable mirror modules in the image scanning device. For moving the movable mirror modules, a transmission mechanism including several auxiliary guide wheels and guide wheels is used to drive the movable mirror modules via a transmission rope. When the driving motor operates, via the transmission rope and the relevant components, the movable mirror modules are driven and moved for optically scanning the document positioned on a document positioning plate. In general, the transmission rope is made of steel material.
FIG. 1 is a perspective view showing a conventional image scanning device 1 comprising a focusing lens 2, a left guide rod 31, a right guide rod 32, a first movable mirror module 4, and a second movable mirror module 5. A transmission mechanism is arranged to drive and move the first and second movable mirror modules 4, 5 via a transmission rope L along the guide rods 31, 32. One end of the transmission rope L is fixed at a locating member 8 which is normally positioned at a suitable position of an inner side wall of the casing of the image scanning device 1.
The transmission rope L is first wound on a guide wheel 51 of the second movable mirror module 5 and then held by a rope holder 41 mounted on an end portion of the first movable mirror module 4. Thereafter, the transmission rope L sequentially travels through a first auxiliary guide wheel 71, a driving motor 6, a second auxiliary guide wheel 72 and a third auxiliary guide wheel 73 and then travels back onto the guide wheel 51 of the second movable mirror module 5. Finally, the transmission rope L travels through a fourth auxiliary guide wheel 74 and then fixed at a tension adjuster 9 which may be positioned on an inner side wall of the casing of the image scanning device or other suitable position. In such arrangement, when the driving motor 6 operates, via the transmission rope L, the first and second movable mirror modules 4, 5 can be driven to move at a predetermined displacement ratio.
The above conventional transmission rope structure has been used for many years in this field. However, in practical application, a deflection and vibration will take place to the movable mirror modules when moving. Many factors resulting in the deflection and vibration of the movable mirror modules have been found by analysis. It is noted that the mechanical tension strain to the transmission rope is a very important factor. The tension strain causes a length deformation to the transmission rope during driving the movable mirror modules. According to the analysis based on technical principle and material mechanics, the tension strain A L of the transmission rope is: ##EQU1## wherein: .DELTA.L: length deformation of the transmission rope L: total length of the transmission rope F: transmission force applied to the transmission rope along the axial direction thereof E: rigidity coefficient of the transmission rope A: area of the transmission rope suffering the driving force.
It is possible to limit the length deformation by means of improving the transmission rope structure so as to solve the problem of the deflection and vibration to the movable mirror modules. FIG. 2 is a plane view showing the arrangement of the transmission rope and the transmission mechanism therefor. It is found that some of the sections of the transmission rope have a fixed length and some of the sections are those sections which will not affect the optical path length, while some other sections have length varying with the displacement of the movable mirror modules. For example, the lengths of the section L1 of the transmission rope between the rope holder 41 of the first movable mirror module 4 and the guide wheel 51 of the second movable mirror module 5 and the section L2 of the transmission rope between the guide wheel 51 of the second movable mirror module 5 and the locating member 8 will vary with the displacement of the two movable mirror modules 4, 5. When the length deformation of the two sections L1, L2 exceeds a predetermined value, the scanning quality of the image scanning device will be greatly affected.